Work in our laboratory has focused on the somatic cell genetics and molecular biology of cell surface antigens. In the past year, we have continued our analysis on the mutants that we have been studying for several years. In terms of the continuing studies on the H-2 antigen mutants, we are in the process of analyzing mutants that have undergone structural alterations in a particular H-2 antigen. We have constructed cDNA libraries from such mutants and are in the process of screening these libraries for the mRNA coded for by the mutant gene. We have also initiated two related projects. One involves the relationship between the expression of beta[unreadable]2[unreadable]-microglobulin and the H-2 antigens. We have observed and are analyzing two interesting mutant cell lines that fail to express H-2 antigens and/or beta[unreadable]2[unreadable]-microglobulin. In one of these mutants, EL/4Mar, the defect appears to be in the H-2K[unreadable]b[unreadable] genes, which are not transcribed. In the other cell line, which we call ABM3, the beta[unreadable]2[unreadable]-microglobulin genes are present and transcribed but apparently are not translated. The conclusion we have been able to draw from our studies on these two mutant cell lines is that the interaction between beta[unreadable]2[unreadable]-microglobulin and H-2 antigens is intricate and defects in the synthesis and expression of either one can result in the failure of the bimolecular complex of H-2 antigens and beta[unreadable]2[unreadable]-microglobulin to appear on the cell surface. The other project we have initiated is to study the somatic cell genetics of beta[unreadable]2[unreadable]-microglobulin. We have been using two cell lines, R8 and 439.4.2, both of which are heterozygous at the B2m locus. Using a monoclonal directed against the product of the B2m[unreadable]b[unreadable] allele, we have isolated a series of mutants that do not express this antigen. The mechanism by which the expression of this antigen is lost in these two cell lines is entirely different. In one cell line, R8, we find that the loss of expression of B2m is due to point mutations in the gene. In the other cell line, 439.4.2, the loss of expression is due to deletions in the gene. All the deletions appear to begin at precisely the same position, and proceed in the 5' direction, resulting in the deletion of the first exon and the promoter. We are attempting to determine the reason for this difference between these two cell lines in the genetic alterations that occur in the B2m gene. (CS)